Apparatus for purifying zinc



March 12, ,1935. L. s. HOLSTEIN Er AL Y 1,994,346

APPARATUS FOR PURIFYING ZINC Filed Feb. 10. 1935 ATTORNEYS Patented Mar. 12, 1935 UNITED STATES PATENT OFFICE M. Gnder, Palmerton, Pa.,

assignors to The New Jersey Zinc Company, New York, N. Y., a corporation ol' New Jersey Application February 10, 1933, Serial No. 656,100

Claims.

This invention relates to purifying zinc vapor and has for its object the provision of an improved apparatus for treating zinc vapor for the purpose of freeing it from contaminating metals.

5 More particularly, the invention involves an improved apparatus for obtaining metallic zinc, zinc dust or zinc oxide of very high purity from relatively impure zinc metal.

Metallic zinc has heretofore been commonly redistilled for the purpose of freeing it from lead and iron. In these heretofore customary processes of redistillation, the removal of lead, in particular, has not been accomplished to the extent that would be expected from the vapor-pressure of lead at the boiling point of zinc.

The decrease in the vapor tension of lead produced by its solution in molten zinc does not, acl

cording to experience, decrease correspondingly the amount of lead actually distilled over in the ordinary commercial practice of redistillation. In such an operation, when carried out at an adequate distilling rate to render the operation profitable, molten zinc carrying lead in solution splashes up against the superheated sides of the distilling retort above the bath of molten metal; the zinc promptly boils off, leaving a film consisting substantially of lead, which tends to establish the true vapor-tension of lead in the vapor in the retort.

The difference in the amount lead actually present in redistilled zinc and the amount which theoretically would be expected is, we believe, largely due to the following factors:

1. Superheatng of zinc vapor-In the ordinary practice of zinc redistillation, the vapor is in contact with and exposed to radiation from the externally heated walls of the retort, or, in the case of internally heated retorts, the vapor is in contact with, and exposed to radiation from, the electric resistor or arc or other means used to generate the heat. The zinc vapor is thus heated to a temperature exceeding the boiling point of zinc. At this temperature the zinc vapor will absorb in vapor form a greater percentage of lead by volume than corresponds to the vapor tension (or partial pressure) of lead at the boiling point of zinc. This excess lead vapor is carried over into the condenser by one or both of the following phenomena, even in cases where the zinc vapor loses its superheat before entering the condenser or the vapor conduit leading thereto.

2. Supersaturation due to lack of equilibrium between vapor phase and liquid phase.-Lead that has been volatilized by superheating as de- (Cl. 26S-19) scribed above will not completely condense out of the zinc vapor when the mixture of zinc vapor and lead vapor has been cooled to the boiling point of zinc. The excess lead (that is, in excess of the amount that would theoretically be present at the existing partial pressure) will in part remain in vapor form without condensing in the absence of molten zinc to dissolve it. and this enables the lead to be carried into the condenser by the zinc vapor, if opportunity be not given for it to establish equilibrium with the molten bath.

3. Entrainment.-Droplets of molten lead or molten zinc mixed with molten lead are formed in the current of zinc vapor either by the condensation of the supersaturated lead vapor or by the mechanical spraying of droplets of molten zinc contaminated with lead into the vapor stream by bursting bubbles of zinc vapor in the bath of boiling zinc. These droplets of molten metal contaminated with lead may be carried mechanically (entrained) into the zinc condenser in the form of a metallic mist by the zinc vapor.

Lead is herein used as a specific example of the manner in which these three factors interfere with the purification of zinc by redistillation. It should be understood that other metals with boiling points higher than that of zinc behave in the same manner and, therefore, are not eliminated from zinc by redistillation to the extent that would be expected from their low vapor tensions at the boiling point of zinc.

An improved method of purifying zinc vapor in which the deleterious influences of the aforementioned three factors are effectively avoided (as a result of which zinc metal of high purity may be obtained) is disclosed and claimed in our copending parent application Ser. No. 540,566, filed May 28, 1931. While that method is especially adapted to the purification of zinc vapor by redistillation, it is equally applicable in principle to the purification of zinc vapor from any source, such, for example, as a zinc smelting or reduction operation in which the zinc vapor is admixed with carbon monoxide gas. In its broad aspect, the method takes advantage of reux condensing to provide an eliminating or purifying agency through or in contact with which the relatively impure zinc vapor passes. Thus a part of the zinc vapor undergoing purification is condensed to molten zinc, as, for example, in a fractionating or refluxing column and this condensed molten zinc passes back to the volatilizing source of the zinc vapor in countercurrent flow to the stream of zinc vapor. In effect, the condensed molten zinc exercises a scouring or scrubbing effect on the relatively impure zinc vapor, and collects a large proportion of the lead, iron and other metals of higher boiling points than zinc and conveys these metal impurities out of the stream of zinc vapor.

The present application is a continuation in part of our aforementioned parent application and is directed to an apparatus for practicing our improved method of purifying zinc. The characteristic feature of this apparatus is its reflux condensing action as a consequence oi which the metal impurities are largely removed from the zinc vapor. Thus, the apparatus of the invention comprises a fractionating column or reflux condenser in which a portion oi' the zinc vapor derived from the distillation retort is condensed and returned to the retort (refluxed) in such a manner that all the zinc vapor leaving the retort is brought in intimate contact with the returning stream (or perhaps shower or mist) of molten zinc. This condensation and return of molten zinc to the distilling retort (or other volatilizing source of zinc vapor) is brought about by conducting the zinc vapor from the retort into an upright (preferably substantially vertical) conduit so constructed and positioned that there is a regulable heat loss or dissipation from its outer surface (by radiation, conduction and convection). A desired amount of heat dissipation from this upright conduit may be provided for by properly proportioning the area of its outer surface (that is the surface through which the heat contained in it must pass to be dissipated), and by surrounding it with heat insulation of suitable thickness. This upwardly extending conduit constitutes the reflux condenser or fractionating column. Suiiicient heat is dissipated from it to recondense part of the zinc vapor evolved' from the volatilizing source. In practice we have found that effective removal of lead (and other contaminants with high boiling points) from the zinc vapor is very satisfactorily carried out if the reflux condenser be so operated as to condense and return to the zinc distillation retort from l5 to 20 per cent. of the zinc vapor evolved therefrom. 'I'he zinc vapor reaching the top of the reflux condenser enters an appropriate conduit leading to any suitable type of zinc vapor treatment device; such as a condenser capable of efliciently condensing large quantities of zinc vapor.

It is advantageous to illl the reux condenser or fractionating column with packing material with the object of rendering the flow of both the zinc vapor and the condensed molten zinc onits way back to the distilling retort tortuous, thereby bringing about intimate and relatively prolonged contact between the molten zinc and the zinc vapor as they move in countercurrent paths. The filling or packing material permits a shorter length of column, and where omitted the reflux condenser or fractionating column must be of sufficient length or height to bring about sufcient contact between the ascending zinc vapor and the refluxed molten zinc to effect the desired result. 'Ihe lling or packing material may consist of any substance that is unaffected by zinc vapor and molten zinc. Thus, lumps of coke, graphite, refractories such as silicon carbide, clay, and the like, crushed brick and the like may be used. 'I'he packing material should be crushed to such a size as to leave adequate voids for the passage of the zinc vapor and molten zinc metal without clogging and creating excessive back prsure. Satisfactory materials used in practice have been coke,4 crushed to about 3/4, and a mixture of crushed refractory brick and reclay, crushed to about 3/4". It is advantageous to use material that crushes with a conchoidal fracture (such as coke or flintclay), since such lumps pile up with point and line contacts instead of surface to surface contacts so that adequate porosity is produced.

Any appropriate form of packing material such as spheres, grids or Raschig rings may, of course, by used. Furthermore, the reflux condenser or fractionating column may itself consist of spiral or tortuous passages or may be equipped with baiiles to produce the desired enect.

The stream of molten zinc descending in the reflux-condenserdecreases the amount of lead carried by the zinc vapor in the following ways:

1. The superheat of the zinc vapor is removed by contact with the molten zinc, the temperature of which does not exceed the boiling point of zinc. i

2. The supersaturated lead vapor condenses when brought into intimate contact with the molten zinc. Y

3. The mechanically entrained droplets of molten lead are rinsed out of the zinc vapor by the molten Zinc.

4. The molten zinc tends to dissolve lead vapor so as to establish the decreased vapor tension of lead produced by its solution in zinc.

The invention may advantageously be considered in connection with the attached drawing in which Figures 1 and 2 set forth diagrammatically apparatus illustrative of the invention.

v This apparatus (see Fig. 1) comprises a redistillation (or spelter) retort 1 of conventional form mounted in an appropriate heating furnace 2 of conventional type. The retort is mounted with a slight forward slope. The front of the retort is in communication with a charging well 3 through an elbow pipe 3', whereby molten zinc metal can be conveniently introduced or fed into theretort. The charging well is equipped with a tap hole 4 on a level with the lowest point of the retort, whereby molten metal enriched in lead may be periodically removed from the retort. Such a charging well and tap hole make it possible to operate the retort continuously through its entire life.

An elbow pipe or vapor conduit 5 communicates with the front of the retort above the pipe 3 and serves to convey the zinc vapor volatilized in the retort into a vertically disposed cylindrical conduit 6 having an opening in the bottom thereof registering and communicating with the upper end of the pipe 5. A transverse perforated plate 6' is arranged within the conduit 6 a short distance above the bottom thereof, and serves to support a column of sized filling material 7. The filling material may advantageously be 'coke crushed to about three-fourthsfinch, or any otherv inert packing -material vsuch as crushed refractory, fire clay brick etc. The top of the conduit 6 is closed by a plate 10 appropriately luted with mud or the like 1l to prevent the escape of zinc vapor. 'I'he conduit 6 is surrounded by appropriate heat-insulating material 9 (such as refuse zinc oxide, dust coal or the like) confined in place by a steel Jacket 8 supported in its operative position in front of the furna'ce in any appropriate manner. The conduit 6 with its packing material 'I constitutes a reflux condenser or fractionating column. 'I'he amount of heatinsulating material 9 surrounding the conduit may be adapted to control the dissipation of heat from the conduit and thus control itsoperating temperature.

A zinc vapor conduit 12 leads downwardly from the top of the column-to an appropriate condenser l3, or other suitable zinc vapor treatment device. The condenser is equipped with an exhaust outlet 14 and a tap hole 15. Both the conduit and the condenser are advantageously heat-insulated in order to condense zinc vapor in an optimum manner.

In the redistillation of zinc in such an apparatus, the temperature in the reflux column is automatically maintained at the boiling point of zinc, provided that the heat radiating surface of the reflux column is properly proportioned. Thus the zinc vapor on its way to the condenser traverses a zone in which any superheat that it may have acquired is entirely removed and in which it is scrubbed by molten zinc at the boiling point of zinc.

The preceding description has referred more particularly to the purification of impure spelter, hard zinc, dross and the like by redistillation. The reflux column may, however, be attached to a retort (see Fig. 2) such as the ordinary horizontal spelter retort producing zinc vapor from ore by reduction. In this case the zinc vapor is admixed with carbon monoxide gas in variable amounts (at least equal to and generally greater than the volume of the zinc vapor) as well as with variable amounts of hydrogen and other gases. In consequence the partial pressure of zinc vapor in the mixture is variable. For this reason the dew point of the zinc vapor varies, with resultant variation in the amount of heat that must be removed from the column to eilfect adequate reflux condensation. Therefore the control of heat removal and temperature is not so` simple in this case as in the redistillation of zinc. It is necessary in this case to adjust the heat radiation from the reflux column to the amount of zinc vapor entering it, for example, by changing the heat insulation. v

While we have hereinbefore described the invention as particularly applied to the purification of zinc vapor prior to its condensation to metallic zinc, the purified zinc Vvapor may be utllizedfor other purposes, as for example, the production of zinc oxide, zinc dust and other zinc products of high purity. Thus, in the production of zinc oxide, the zinc vapor treatment device may comprise means for bringing the purified zinc vapor in contact with an oxidizing gas, such as air, whereby the zinc vapor is burned to zinc oxide, that may be appropriately collected in a manner well known to the art. Similarly, in the production of zinc dust, the zinc vapor treatment device may comprise means, such as a canister, for condensing the zinc vapor to zinc dust; also in a manner well known to the art.

Various modifications may be made in the construction of the reflux column. Its heat radiating surface should be of sufficient area to effect the refluxing of a certain proportion (say around 10 to 30% in the case of lead elimination) of the zinc vapor entering the column. With a distillation retort of the spelter type (approximately 5 feet long and 7-10 inches internal diameter) excellent operating results have been secured with a conduit (6) two feet three inches high and 'l1/2 inches internal diameter with a wall thickness of one inch. The steel casing 8 was 121/2 inches in diameter and the annular space between the cylinder and casing was filled with refuse zinc oxide. It is to be understood that these dimensions are given solely for the purpose of illustration and are in no sense restrictive of the invention.

'I'he apparatus of the invention may be used in purifying zinc of cadmium and the like in accordance with the method disclosed and claimed in the copending application of Messers. Ginder, Peirce and Waring, Serial No. 620,634, led July 2, 1932. When so used, the metallic vapor reaching the top of the column is rich in cadmium and the refluxing molten metal is the refined or puriiled zinc product.

We claim:

1. In an apparatus for purifying zinc, the combination comprising a zinc distillation retort, a charging well associated with the said retort, a reflux column, and a condenser for collecting in the form of molten zinc the zinc not refluxed in the reux column.

2. An apparatus for purifying zinc according to claim l, in which the charging well is provided with a tap-hole situated below the bath of molten zinc in the distillation retort, so that metallic residues may be tapped out of the retort.

3. Apparatus for purifying zinc comprising a substantially horizontally disposed elongated retort, heating means operatively associated with said elongated retort, a reflux column, a conduit connecting an end of the elongated retort with the lower end portion of the reflux column, means for introducing unvaporized zinc into an end of the elongated retort, and a condenser connected with the upper end portion of the reux column.

4. Apparatus for purifying zinc comprising a substantially horizontally disposed elongated retort, heating means operatively associated with said elongated retort, a reflux column, a conduit connecting an end of the elongated retort with the lower end portion of the reflux column, means for introducing unvaporized zinc into the elongated retort, said means being disposed adjacent said conduit, and a condenser connected with the upper end portion of the reflux column.

5. Apparatus for purifying zinc comprising a substantially horizontally disposed elongated retort, means for introducing unvaporized zinc into said elongated retort at an end thereof, heating means operatively associated with said elongated retort, a heat insulated reflux column having interior walls refractory to molten zinc, packing refractory to molten zinc disposed within the reflux column in such a manner that a plurality of tortuous paths are provided for ascending zinc vapor and refluxing molten zinc, a conduit connecting an end of the elongated retort with the lower end portion of the reflux column whereby ascending zinc vapor may enter the reflux column and descending refuxed molten zinc may flow by gravity into the elongated retort, and a condenser connected with the upper end portion of the reflux column.

LEON S. HOLSTEIN. PHILIP M. GINDER. 

